Transmission network



Patented Apr. 3, 1934 PATENT OFFICE TRANSMISSEON NETWORK William It. Bennett, Jersey City, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. EL, a corporation of New York Application July 25, 1931, Serial No. 553,095

5 Claims.

This invention relates to wave transmission networks and more particularly to transmission equalizing networks for use in connection with very long transmission lines such, for example, as a trans-oceanic telephone cable. It has for its principal object improving the signal to noise ratio of currents received over transmission lines of this type.

In very long telephone circuits, in which for physical or other reasons repeaters cannot be used, the speech currents suffer an extremely great attenuation and, even with large input power, are received at a very low intensity at the output end. The attenuation moreover is not uniform but increases with frequency so that a large degree of amplitude distortion results.

At the receiving end of such a circuit the problems of equalization and amplification present great difficulties. If energy amplification is provided sufiicient to bring the greatly attenuated high frequencies up to a suitable level, the lower frequencies, being much less attenuated in the cable, are increased so greatly in magnitude as to cause overloading of the amplifier and resulting distortion of speech. Moreover, since a large number of stages of amplification may be necessary to increase the high frequency components to the desired level, tube noises are introduced which may be sufiicient to mask these frequencies. Preliminary equalization by attenuating the low frequencies to the same degree as the high frequencies serves to prevent overloading, but brings all of the signal components down to a level comparable with that of tube noises. Moreover, the use of resistive equalizers for this purpose introduces a further noise component due to the thermal agitation of the electrons in theresistance elements and still further reduces the signal to noise ratio of the received currents.

It has been found that the use in combination with a voltage responsive amplifier of a preliminary equalizer containing only reactance elements permits the voltages of the higher frequency signal components to be increased without affecting the initial signal to noise ratio. The number of amplifier stages required for the subsequent amplification of the signal is also diminished and the tube noises are correspondingly reduced. This method of equalization depends primarily on resonance effects in the reactive equalizer whereby the voltages of the higher frequency signal components are selectively increased. To make use of it in cases where a large degree of equalization is needed over a wide range of frequencies a problem presents itself due to the sharply selective character of resonant circuits. It is possible with simple resonant circuits to obtain very high amplification of a single frequency or of a group in a narrow frequency range, but frequencies outside this range may be amplified so little that no useful equalization results. rdinary combinations of resonant circuits tend to produce multiple resonance peaks in the equalization characteristic which are detrimental to the transmission.

In accordance with this invention a reactance type equalizer is provided which provides a high egree of amplification of the high frequency signal currents together with a proportionately large amplification of the lower frequency currents. In its general form the equalizer of the invention comprises a plurality of reactive networks coupled together in tandem to form a tapered artificial line. In the preferred form each section of the equalizer is an all-pass net work and all sections have the same propagation constants but different characteristic impedances, the latter increasing in value in a prescribed manher from one end of the line to the other. The equalization characteristic may be regarded as being due to the summation of the reflection effects at the junctions of the sections and at the ends of the line and may be modified by varying the characteristic impedances of the component sections. 5

The invention will be more fully understood from the following detailed description and by reference to the drawing of which Fig. 1 illustrates schematically a network in accordance with the invention; 90

Fig. 2 shows curves illustrating typical characteristics of the network of Fig. 1; and

Figs. 3 and 4 illustrate alternative forms of the networks of the invention.

The equalizer shown in Fig. 1 comprises 12 sections of all-pass lattice networks in which the line impedances are inductances and the lattice impedances are simple capacities. At one end the network is connected through a resistance R to a source of oscillations having an E. M. F. E, this combination representing a cable or long transmission line with which the equalizer is to be used. At the other end the network terminals are connected directly to the grid and cathode respectively of an amplifier A which may be the first stage of a multi-stage receiving amplifier, The amplifier A provides a virtual open circuit termination for the network.

Taken in their order from the input end of the line, the inductances of the lattice sections have the respective values:

0L1L0, azLo, ttnLo, (1)

and the capacities have the values where the as represent numerical coefficients. From the well known formula relating to lattice networks it follows that the characteristic impedance and the propagation constant or" the r section are given by and where and P denote respectively the characteristic impedance and the propagation constant, and f denotes frequency, The characteristic impedance is constant and resistive, the value being proportional to the numerical coefiicient (Zr. The propagation constant is evidently the same for all sections and is a pure imaginary quantity.

This corresponds to the fact that the attenuation constant, which is defined as the real part of the propagation constant, is zero at all frequencies. Networks of this type are adapted. to transmit currents of all frequencies substantially without attenuation and have been termed all-pass networks. The general type of four-terminal reactance network has a propagation constant which is real in certain frequency ranges and imaginary in others, the real values occurring in'attenuation ranges and the imaginary values inranges of free transmission. If Lu and C0 are and ifthe resonance frequency indicated bythe product LuCobe denoted by f0, Equations (3) and (4) become and P ,f tanh J (6) If the a coefiicients were all unity the-network charactcristicimpedance would be equal to the resistance R of the connected line and as a result of this impedance match waves of all frequencies would be transmitted to the amplifier without change of amplitude. To provide a variable amplification of the output E. M. F. the a coefficients of the successive sections taken in order from the input end are given increasing values so that the characteristics impedances of the several sections increase progressively towards the amplifier terminals. A wide variety of voltage characteristics is obtainable by appropriate selection of the impedance values, but it has been found that the-most satisfactory characteristics for the purpose of equalizing the attenuation in long telephone cables are obtained by stepping up the characteristic impedances only a small-amount in the sections at the input end and rapidly increasing the step-up ratio in the successive sections. 'By this arrangement it is possible to obtain a voltage amplification which, measured in decibels, increases in a nearly linear fashionwith sections. voltage ratio variation obtainable with a single voltage ratio in the general case can be expressed where E2 denotes the output voltage, E the line voltage, and n the number of sections in the equalizer, the values of the a coeiiicients determining the characteristic impedances are found to be as indicated by the following table:

T s e c t io rl s "P a3 he form of the voltage characteristic is shown by curves 1 and 2 of Fig. 2 in which the abscissa are proportional to the frequency and the ordinates to'the logarithm of the voltage ratio Z/E. Curve 1 corresponds to an equalizer of five sections and curve 2 to one having four For comparison, curve 3 shows the resonant circuit proportioned to give a maximum gain of 25 decibels.

The method of calculatingthe coefficients in the above case depends upon thefact that the and N2n'iS a polynomial in X of degree 211, the

'coefiiciente of the terms of which are functions of the a coefiiciencts of the several network sections. By-identifying the absolute value of the square of the polynomial N22 with the expression 1 -1- IXI n in accordance with the procedure discussed in my cop-ending application Serial No. 374,669, filed June 29, 1929, a set of equations can be obtained which are suiiicient for the determination of the impedance coefiicients.

It \vill'be noticed that where the number of sections exceeds two, the characteristic impedance of the first few sections is practically the same as theline impedance R, the a coefficients being very close to unity. Since the impedance mismatching at the junctions of the initial sections is in thesecases so slight it is evident that only slight modification of the characteristics would result from their omission. For

practical purposes the network neednot generally her)" the method of calculation in this case following along lines similar to those indicated above. The values of the a coefficients for numbers of sections up to four are given in the table below.

I E2 is.

s ytio s a2 The single-section network in this case gives no voltage amplification and therefore is not of interest. Likewise in the three-section network the first section may obviously be omitted.

The voltage amplification obtained by a foursection network of this type is indicated by curve 4 of Fig. 2. In the lower frequency range the characteristic is about the same as that of the four-section network corresponding to curve 2, but at higher frequencies the slope of the characteristic is more gradual and the maximum gain is less. The gain at intermediate frequencies is higher in pro-portion to the maximum gain than in the case of curve 2.

While the proportions given in the foregoing tables provide highly desirable characteristics, other proportions may also be used and useful characteristics may be obtained therewith. Dotted curve 5 of Fig. 2 represents the voltage amplification characteristic of a two-section network for which 111:3.0'7 and a2=17.8. For a large part of the frequency range the amplification is not greatly different from that of the five-section network corresponding to curve 1, although the variation is somewhat less regular. By following the plan of increasing the impedance mismatch at the successive junction points the general form of the characteristic is maintained and by varying the degree of mismatching at the different junction points the shape of the characteristics may be varied to a large extent.

Networks of the type shown in Fig. 1 are characterized by a voltage amplification characteristic having a single maximum occurring at the resonance frequency in. By the use of more complex impedances in the branch arms, characteristics having more than one maximum can be obtained. For example, if each section consists of an all-pass network of the type shown in Fig. 3, a characteristic having two maxima is obtained. In this network the line and the lattice impedances of the r section are designated (M21 and cab respectively, Z1 and Z2 being series resonant and parallel resonant circuits respectively, tuned to the same frequency and proportioned reciprocally so that The voltage characteristic of this type of network can be made to follow variations represented by formulae similar to Equations (7) and (8), namely if the a coefficients are given the values in the tables above corresponding to Equations (7) and 1 (8) respectively. The function X in this case has the form where f1 and is are the two resonance frequencies of the impedance constituted by a line branch and a lattice branch in series. Since X is unity at each of these frequencies they correspond to maxirna in the voltage characteristic.

The lattice type networks of Figs. 1 and 3 being of the balanced type are not well adapted for use in systems having one side of the line grounded. In such systems the balanced lattices may be replaced by their unbalanced equivalents, for example, the lattices of the network of Fig. 1 may be replaced by networks of the type shown in Fig. 4. In this network, which corresponds to the 'r section of Fig. 1 there are included in one side of the line two inductances each equal to EZ L 2 which are coupled together with unity coupling in series aiding relation from terminal B to terminal B. At the mid-point a capacity is connected across the line.

Other types of unbalanced equivalents of a lattice which may be used in place of the network of Fig. 3 are shown in United States patent to H. Nyquist 1,770,422, issued July 15, 1930.

In the appended claims the term all-pass network is used to define a four-terminal reactance network having an attenuation constant which is substantially zero at all frequencies.

What is claimed is:

1. A frequency selective transmission network adapted to operate between a wave source having relatively low impedance and a receiving device having a high input impedance comprising a plurality of all-pass reactance networks connected in tandem, said reactance networks having the same propagation constants and having characteristic impedances which increase in value progressively from one end of the transmission network to the other.

2. A frequency selective transmission network adapted to operate between a wave source and a receiving device having a very high input impedance comprising a plurality of all-pass reactance networks connected in tandem, said reactance networks having the same propagation constants and having characteristic iinpedances which increase in value progressively and at an increasing rate from one end of the transmission network to the other.

3. A frequency selective transmission network adapted to operate between a wave source of relatively low impedance and a receiving device of very high impedance comprising a plurality of all-pass reactance networks connected in tandem, said reactance networks having character- .istic impedances which increase in .value progressively from one end of the network to the other and having equal propagation constants, the

.propagation constant and-the relative values of the characteristic impedances being proportioned to provide a voltage transfer characteristic which increases approximately uniformly throughout a preassigned frequency range.

,4. In combination, a wavesource of relatively low impedance, a receiving amplifier having a very high input impedance and a transmission network coupling said source to the inputterminals of said amplifier, said transmission network comprising a plurality of all-pass reactance networks connected in tandem having equal propagation constants and having characteristic impedances which increase progressively in value from stage to stage towardsthe end of the transmission network connected to the amplifier wherebyavoltrage :transfer characteristic is obtained which'innetwork has a voltage amplification charactera istic adapted to compensate the attenuation of the telephone line in 'the speech range.

WILLIAM R. BENNETT. 

